1. Field of the Invention
Embodiments of the present invention relate to the field of capacitive power supply circuits which are intended to provide a rectified smoothed output voltage across a power storage element (capacitor) from an alternating current (A.C.) supply voltage.
2. Discussion of the Related Art
Capacitive power supply circuits belong to the different solutions for supplying power to a load from an A.C. supply voltage originating, for example, from the electric supply mains (220 volts or 110 volts).
Capacitive power supplies are especially preferred to magnetic transformer or high frequency power supplies (switched-mode power supplies) for small powers (output currents of approximately some ten milliamperes) for reasons of cost or when the stand-by consumption is desired to be minimized.
FIG. 1 shows a conventional example of a capacitive power supply circuit This circuit essentially comprises a first capacitor C having a D.C. output voltage Vout provided between its terminals 1 and 2. This voltage is obtained from an A.C. supply voltage Vac (for example, the electric distribution mains voltage) applied between two input terminals 3 and 4 of the capacitive power supply circuit terminals 2 and 4 being connected. Between terminals 3 and 1 are connected, in series, a resistor R1, a capacitor C1, and a diode D1 having its cathode directly connected to terminal 1 (positive electrode of output capacitor C). Diode D1 forms a halfwave element for rectifying voltage Vac to charge capacitor C. The value of output voltage Vout is set by a zener diode DZ1 connecting the anode of diode D1 to ground 2 (the cathode of diode DZ1 being directly connected to the anode of diode D1). The function of resistor R1 is to limit the current surge on circuit power-on. This resistor is sometimes omitted. The function of capacitor C1, which is a high-voltage A.C. transistor (several hundreds of volts), is to limit the current provided to the load. Diode DZ1 is used for the regulation while capacitor C, which is a low-voltage capacitor (a few tens of volts at most), is used as a power sink. Diode D1 is used to prevent the discharge of capacitor C into the A.C. power supply. Generally, resistor R1 is a normalized resistor and the most currently used output voltage is an output voltage of from a few volts to a few tens of volts.
The output current of the power supply circuit essentially is a function of the value of capacitor C1 (and of voltage Vac, frequency and amplitude). Accordingly, capacitor C1 is selected according to the load to be supplied.
As long as voltage Vout has not reached the threshold voltage of diode DZ1 (neglecting the voltage drop in diode D1), diode DZ1 is blocked, enabling charge of the capacitor in positive halfwaves of voltage Vac. As soon as voltage Vout reaches value DZ1, the zener diode starts to avalanche, interrupting the charge of capacitor C.
Such a capacitive power supply has the advantage of an easy implementation as compared to other magnetic transformer or high-frequency solutions.
However, for output currents greater than some ten milliamperes, it generates significant losses when the system is at stand-by, that is, when no power is sampled by the load connected to terminals 1 and 2.
The significant losses during system stand-by result in that, in practice, the capacitive power supply circuits are limited to applications of supply of a current on the order of some ten milliamperes.
To solve this problem, an A.C. power supply circuit using a controllable switching element has already been provided.
FIG. 2 shows an example of such a circuit.
As compared to the assembly of FIG. 1, zener diode DZ1 is replaced with a cathode-gate thyristor Th forming a controllable switch. Thyristor Th connects the anode of diode D1 to ground (common terminals 2 and 4), the cathode of thyristor Th being grounded. The gate of thyristor Th is connected by a zener diode DZ2 to output terminal 1, the anode of diode DZ2 being connected to the gate of thyristor Th. Finally, a diode D2 is connected in antiparallel with thyristor Th, its anode being connected to the terminal 4 while its cathode is connected to the anode of diode D1. Functionally, thyristor Th is intended to be on when capacitor C needs not be charged (voltage Vout greater than the threshold voltage of diode DZ2) and to be off when a charge of capacitor C is required.
FIGS. 3A, 3B, and 3C illustrate the operation of the circuit of FIG. 2 and show, respectively, examples of shapes of voltage Vac, on periods (ON) of thyristor Th, and on periods (ON) of diode D1 for the charge of capacitor C. As long as voltage Vac is, in a positive halfwave with the orientations of the drawings, smaller than threshold voltage VZ2 of diode DZ2, thyristor Th is off and diode D1 is on. From the time (time t1) when voltage VZ2 is reached by voltage Vac, thyristor Th turns on, which forbids continuing the charge of capacitor C. The load connected to terminals 1 and 2 is then supplied by the discharge of capacitor C and diode D1 is off. During positive halfwaves, diode D2 is off.
Towards the end of the positive halfwave (time t2), when voltage Vac falls below threshold VZ2, thyristor Th blocks. However, since voltage Vout is then in principle greater than voltage Vac, diode D1 remains off. Diode D1 is however likely to turn back on by the turning back off of thyristor Th between times t1 and t2 if the load has consumed all the power stored in capacitor C. This hypothetical case is however rather unusual since capacitor C is sized according to the load that it must supply.
From the beginning of the negative halfwave (time t3) and until the next zero crossing (time t0) towards the positive halfwave, diode D2 is forward biased and forbids Conduction of thyristor Th.
The losses in the circuit of FIG. 2 result, in positive halfwaves, from the current in thyristor Th and, in negative halfwaves, from the current in diode D2.
The losses in negative halfwaves in diode D2 approximately correspond to the losses in diode DZ1 (FIG. 1). However, in positive halfwaves, the losses in thyristor Th are much smaller than those in diode DZ1 of FIG. 1. Indeed, the current in stand-by periods, that is, when voltage Vout remains greater than threshold voltage REF since the load does not consume, is a D.C. current in thyristor Th, and thus under a voltage on the order of one volt while, in the case of FIG. 1, it is the avalanche voltage of the zener diode (10 volts, or even more).
For a same admissible system stand-by loss level, the circuit of FIG. 2 enables supplying loads with much greater currents (typically of several tends of milliamperes under a few tens of volts) without for all this increasing the stand-by consumption.
However, a disadvantage of the circuit of FIG. 3 is that it poses problems of electromagnetic compatibility and requires use of a mains filter (not shown) upstream of the system.
A circuit such as described in relation with FIG. 3 is described, for example, in U.S. Pat. No. 5,796,599.